This invention relates generally to plates and fasteners which are employed for securing insulation and the like to roof decks. More particularly, the present invention relates to threaded fasteners which engage stress plates for fastening insulation and like material to roof decks.
In roof systems to which the present invention relates, threaded fasteners engage metal plates or plastic plates of various configurations to secure insulation material to a metal roof deck. Plies of synthetic coating, tar, gravel or various roofing materials are then applied over the insulation to complete the roof. Most of the plates employed for such roofing applications, whether they be composed of metal or plastic, have a pre-fabricated central opening which receives the fastener. The plates have a generally planar portion for engaging the insulation along a substantial surface area. The plates are sufficiently rigid to secure the insulation to the deck under intense wind-produced lift forces which may be exerted on the roof.
Plastic and/or molded stress plates have in many instances replaced the conventional metal plates. Among a number of advancements in plate/fastener technology are improved provisions for aligning the fastener as it penetrates the insulation and metal deck, improved structures for preventing the plate from being pulled over the fastener, improved moisture repellent characteristics, and improved structures to lessen the tendency of the plate to cut or penetrate the plies of material installed over the insulation.
Both metal and plastic stress plates are susceptible to the fastener being overdriven (excessive penetration depth into the anchoring deck) resulting in an excessive load being exerted on the plate. While installation tools for driving the fasteners ordinarily employ depth setting mechanisms which are directed to prevent overdriving of the fastener, such tools require a critical depth adjustment necessitating periodic field adjustments. In a number of installation projects either no depth adjustment tool is employed and/or the installation tool is not properly adjusted for the specific roof construction.
U.S. Pat. No. 4,361,997, invented by the inventor of the present invention and entitled, "Fastener Plate and Assembly", discloses a plastic plate employed in combination with a threaded fastener to secure insulation to a metal roof deck. The plate has a planar portion with a centrally disposed tapered hub. A central bore through the hub is dimensioned to facilitate proper alignment of the fastener as it is driven through the insulation into the roof deck so that a perpendicular orientation of the fastener to the deck is maintained. The head of the fastener is seated in a counterbore of the plate below the upper surface of the plate when the proper depth is obtained, thereby clamping the insulation to the deck. In such conventional fastening systems, the installation process must be essentially completed simultaneously with substantially the seating of the fastener in the counterbore of the plate.
A disadvantage of the fastener/plate assembly, such as disclosed in U.S. Pat. No. 4,361,997, as well as other assemblies wherein the fastener seats onto or within the stress plate, is the absence of effective means for tolerating fastener overdriving which can and does frequently occur during installation. Should the fastener be overdriven, the fastener head would force the plate into the insulation, potentially developing an excessive load on the plate. For a relatively soft and yielding insulation, the increased load exerted on the plate due to overdriving may not be sufficient to cause the plate to fail. However, if the insulation is relatively rigid or non-compressible (high compressive resistance), the load could result in stress cracking of the plate. The resistance to joint loosening may also be lost in overdriving the fastener. The threads which are formed in the plate could strip out so that the fastener loosens from the plate.
Over extended time periods, vibratory forces also tend to loosen the fastener. The fastener may pop above the plate and/or the plate may transform to a "reverse umbrella" configuration. In either case, the integrity of the roofing plies is threatened. Thus, the advantages of conventional fastening systems as discussed may even be negated over time by proper driving of the fastener.